Wavelength conversion laser light sources have been developed and commercially available which convert light (fundamental waves) emitted from laser media such as Nd:YAG lasers and Nd:YVO4 lasers into visible green light (harmonic waves), and, further convert the green light into ultraviolet light by wavelength conversion by a nonlinear optical effect. These visible laser light and ultraviolet laser light are used in various applications, such as laser beam machining of materials and light sources for laser displays.
Nonlinear optical crystals having birefringence need to be used to achieve the nonlinear optical effect, which crystals are manufactured by periodical polarization inversion of ferroelectric nonlinear crystals such as LiNbO3 (lithium niobate: PPLN).
FIG. 1 is a schematic view illustrating the outline configuration of a wavelength conversion laser light source. FIG. 1 shows the configuration example of an end-pump laser light source that receives excitation light from an end face of a laser medium.
As shown in FIG. 1, wavelength conversion laser light source 10 includes excitation light source 11, collimator lens 13, collecting lens 14, solid-state laser medium 15, wavelength conversion element 16, concave mirror 22, and optical reflection film 23.
Solid-state laser medium 15, wavelength conversion element 16, and concave mirror 22 build up laser resonator 24.
Wavelength conversion element 16 is an SHG (second harmonic generation) element, which converts the wavelength of fundamental wavelength laser light (infrared laser light) of a 1064 nm wavelength outputted from solid-state laser medium 15 to generate half-wavelength laser light (green laser light) having a 532 nm wavelength.
Wavelength conversion laser light source 10 excites solid-state laser medium 15 with excitation light source 11, and converts the generated near-infrared light to green laser light with wavelength conversion element 16.
Excitation light 12 is emitted from excitation light source 11, is collimated by collimator lens 13, and then is focused onto solid-state laser medium 15 building up laser resonator 24 through collecting lens 14.
Solid-state laser medium 15 is a YVO4 crystal, which is a single crystalline material. Solid-state laser medium 15 has an end face (optical reflection film) 18 which the excitation light is incident on. On the end face, high reflective optical film 18 is formed that reflects light of a 1060 nm band. High reflective optical film 18 functions as a resonator. High reflective optical film 23 that reflects the light of a 1060 nm band is also formed on an end face of concave mirror 22. Concave mirror 22 also functions as a resonator.
End face 19 of solid-state laser medium 15 and end face 20 of wavelength conversion element 16 face each other and are provided with respective non-reflective optical films. That is, the non-reflective optical films are formed on the face, opposite to wavelength conversion element 16, of solid-state laser medium 15 and on the face, opposite to solid-state laser medium 15, of wavelength conversion element 16, respectively. Laser resonator 24 operates as an optical resonator that resonates light between high reflective optical films 18 and 23 formed on the end faces of solid-state laser medium 15 and concave mirror 22, respectively, to cause oscillation of laser light of a 1060 nm band.
At this moment, the oscillating light of a 1060 nm band passes through wavelength conversion element 16 to be converted to light with an approximately 530 nm wavelength (green light), i.e., half-wavelength. Converted harmonic wave light (green light) 17 of 530 nm is then outputted from end face 21 of wavelength conversion element 16.
Wavelength conversion element 16 is composed of, for example, lithium triborate (LiB3O5:LBO), which is a dielectric single crystalline material, potassium titanyl phosphate (KTiOPO4:KTP), magnesium-doped lithium niobate (Mg:LiNbO3) having a periodic polarization inversion structure, and magnesium-doped lithium tantalate (Mg:LiTaO3) having a periodic polarization inversion structure.
Among them, magnesium-doped lithium niobate having a polarization inversion structure, which has a large nonlinear optical constant, takes full advantage of the large nonlinear optical constant by the polarization inversion structure. The magnesium-doped lithium niobate having the polarization inversion structure has a benefit of inhibiting a change in refractive index depending on light (light damaging) by magnesium ions.
As a result, the magnesium-doped lithium niobate having the polarization inversion structure used as a wavelength conversion element can function as a green laser light source with high output and high efficiency.
Ideally, the ratio of an inversion region to a non-inversion region of the polarization inversion structure should be 1:1 in order to achieve high efficiency of the wavelength conversion element.
Patent Literatures 1 and 2 describe magnesium-doped lithium niobate, magnesium-doped lithium tantalate single crystal, and an optical functional element prepared a special crystal deposition system, where the crystal has a specific crystal composition, i.e. a mole fraction Li2O/(Nb2O5+Li2O) (stoichiometric composition) of 0.49 to 0.52. They also describe an attempt to decrease an applied voltage to form the polarization inversion structure and to provide an ideal ratio of the inversion region to the non-inversion region of the polarization inversion structure using single crystals of the magnesium-doped lithium niobate and magnesium-doped lithium tantalate.
Patent Literature 3 describes a method of manufacturing a wavelength conversion element that outputs stable harmonic waves even during a long-time operation by reducing a change in phase matching temperature. The method of manufacturing the wavelength conversion element disclosed in Patent Literature 3 involves formation of a polarization inversion layer and heat treatment after removal of electrodes, where the temperature of the heat treatment is 85° C. or less.